This section provides background information related to the present disclosure which is not necessarily prior art.
Angular position encoders are measuring systems for a rotating shaft and are used, for example, in the rotors of a wind turbine. Their purpose is to determine the rotor position and/or speed of such a system, wherein the rotor of a wind turbine has a rotor hub with perpendicularly protruding rotor blades and a rotor shaft, which is mechanically coupled to a generator for power generation.
An angular position encoder has a scanning head arranged on the rotating shaft and detects or measures by means of a sensor array arranged in the scanning head, working optically, electrically or magnetically, the position of the shaft. From the sensor signals identified, by means of a measurement value evaluation, actual positional values of the shaft are generated. By means of a time measurement from the positional measurements actual rotor speeds can be identified. The positional and speed data identified in this way are passed to a system controller, which generates from them control and/or regulation signals for the wind turbine. The positional data is required to have a very high degree of accuracy, since measurement errors lead to incorrect control and regulation values for the system.
An angular position encoder arrangement referred to as an angular measurement system is disclosed, for example, in DE 298 15 905 U1. The known arrangement has a materialized measure in the form of a disc on the outer circumference of a shaft, on the circumference of which a measuring element is shown as a materialized measure, which according to the scanning principle meets the requirements for normal optoelectronic, magnetic, inductive or capacitive scanning heads.
The leaflet from the Baumer Group, “Baumer, Sensorama, Innovation and Technology”, issue January 2011, shows an arrangement referred to as a “magnetic tape encoder”, which discloses a materialized measure in the form of a magnetic tape, which is scanned by a scanning head. The magnetic tape encoder is strapped onto a shaft by means of a buckle like a waist belt. Two sensors integrated into the scanning head detect the magnetic field of the tape and in an evaluation unit generate a measurement signal, from which via an arc tangent function positional values for the shaft are identified. The scanning takes place without interruption over the buckle and over any breaks in the materialized measure, without signals or pulses being lost. The known magnetic tape encoder allows retrofitting, in particular in larger shaft diameters, such as for example those of the rotor shaft of a wind turbine.
The known angular position encoders have considerable disadvantages, however, in particular when used on the rotor shaft of a wind turbine. On the one hand they must be highly resistant to vibrations and also operate reliably under extreme environmental influences such as for example those of the high seas in offshore use. At the same time, however, high measurement accuracy in determining the position is necessary in order to achieve the levels of measurement accuracy mentioned above. With the magnetic materialized measure, there is also a danger that metallic foreign bodies or strong magnetic fields will damage the tape and the measurement results will be falsified.
Conventional angular position encoders meet these requirements only to a limited extent, since in particular the recording of the measurement values from the magnetic tape encoders during heavy duty wind power operation is very difficult so that such angular position encoders are very expensive.
Furthermore, the measurement value recording has major sources of error due to inadequate recalibration, synchronization and measurement value resolution. For the control of wind turbines in the offshore area, for example, scanning rates of the angular position encoder of approximately 10-12 bits are normal. This corresponds to a measurement accuracy of approximately 0.1-0.2 degrees in relation to the shaft diameter.